1. Field of the Invention
The invention relates to a compression-type refrigeration system of a motor-vehicle air-conditioning unit having a supercritical process, a refrigerant being circulated in the compression-type refrigeration system via refrigerant lines from a compressor via a gas cooler, an expansion member and an evaporator back to the compressor. Furthermore, the invention relates to a method of operating a corresponding compression-type refrigeration system.
2. Description of the Related Art
Compression-type refrigeration systems in modern motor-vehicle air-conditioning systems are directly driven by the internal combustion engine of the vehicle via a belt drive. As a result, the rotational speed of the compressor driven by the drive belt is directly related to the rotational speed of the internal combustion engine. However, the rotational speed of the internal combustion engine varies greatly under the various operating conditions of the vehicle. Thus, for example, a high rotational speed will prevail during high-speed expressway travel, whereas low revolutions will predominate during town travel or in particular also in a backup. Therefore it is precisely at slow speed, during which the required refrigerating capacity of the air-conditioning unit is greatest as a rule, that the lowest rotational-speed level is available. In order to ensure control of the refrigerating capacity in these units independently of the rotational speed of the internal combustion engine, these units have compressors with a variable swept volume. These compressors are, for example, so-called swash-plate compressors. These units have several disadvantages, such as, for example, the requisite increase in the idling speed of the internal combustion engine in order to prevent the same from stalling or the decrease in the volumetric efficiency when turning down the unit in a controlled manner on account of the back expansion volume related to the design. A decrease in the efficiency has an adverse effect in particular on the energy consumption.
In order to avoid the aforesaid disadvantages, electrically driven compression-type refrigeration systems for motor vehicles, such as, for example, in DE 37 31 360 A1, have already been proposed. In electrically driven compression-type refrigeration systems, the refrigerating capacity can be directly adapted to the value currently required for cooling the vehicle interior space. In particular, subsequent heating of the cooled air cannot occur. Electrically driven compression-type refrigeration systems therefore provide a good way of optimizing the primary energy input. However, additional problems are associated with the new type of drive. In particular, the problem of the generation of heat at the electric motor and of the motor control may be mentioned. It is therefore proposed in DE 37 31 360 A1 to sequentially override or disregard the comfort settings selected by the driver in order to avoid overloading of the electric motor. In other words, this means that the temperature at the electric motor is determined and the unit is turned down in a controlled manner if a critical limit value is reached. By this measure, however, a reduction in the refrigerating capacity will possibly be carried out precisely when a high refrigerating capacity is required. In addition, refrigeration systems with a supercritical process are not mentioned in DE 37 31 360 A1. The expression supercritical process is used when no gaseous/liquid phase transition takes place between the compressor outlet and the inlet of the expansion member. Such a supercritical process occurs, for example, when CO2 is used as the refrigerant.
The object of the invention is therefore to specify a compression-type refrigeration system with a supercritical process in which said disadvantages are avoided and which at the same time has a high efficiency and thus a low primary energy input.
This object is achieved in a compression-type refrigeration system of the generic type in that the compressor is driven by an electric motor with motor-control device, and that the motor and/or the motor-control device is arranged in such a way that the dissipation of the heat loss of the motor and/or of the motor-control device is effected essentially via the refrigerant of the compression-type refrigeration system.
By the dissipation of the heat loss of the motor and/or of the motor-control device via the refrigerant, very efficient cooling of these elements is achieved. In the region between the outlet of the expansion member and the inlet of the compressor, the refrigerant has a temperature of only a few degrees Celsius. Both the temperature of the motor and the temperature of the motor-control device can therefore be reduced by the measure according to the invention to a temperature level at which the losses of the motor and of the motor-control device are substantially lower than at the high temperature levels which normally prevail in the engine compartment. Due to the low temperature level, the resistance of the motor windings, for example, is thus markedly reduced. The primary energy input may therefore already be reduced solely by the reduction in the operating temperature of the motor and/or of the motor-control device, since less heat loss occurs.
It was recognized that, due to the heat loss of the motor and/or of the motor-control device, this heat loss being supplied to the refrigerant, a decrease in the efficiency of the compression-type refrigeration system may occur in an otherwise unaltered complete system. By the supply of additional heat to the refrigerant, the superheating of the refrigerant at the inlet of the compressor can be increased. Furthermore, due to an increased compressor outlet temperature, the pressure and the specific enthalpy at the gas-cooler inlet are now increased, so that the efficiency of the unit decreases as a result of this process. In order to reliably avoid this undesirable effect, means for setting the superheating of the refrigerant at the compressor inlet are therefore also preferably provided. This achieves the effect that the superheating at the compressor inlet does not exceed the desired value. In particular, the absorption of the waste heat of the motor and/or of the motor-control device is taken into account in the process. To set the superheating at the compressor inlet, in particular a thermostatic expansion valve or an electronically controlled expansion valve may be provided. In this case, the superheating of the refrigerant at the compressor inlet is set by controlling the rate of flow through the valve and thus the refrigerant mass flow. At the same time, according to the invention, the reference superheating for the control of the expansion member is tapped directly at the compressor inlet. As a result, the effect of the waste heat of the motor and/or of the motor-control device is automatically taken into account.
In a preferred arrangement, the electric motor is accommodated together with the compressor in a common housing. This arrangement has the advantage that leakages at a rotating shaft, which may occur in an arrangement of the electric motor outside the compressor housing, are avoided from the beginning. In this case, especially effective cooling of the electric motor is achieved if the refrigerant flows directly around the electric motor. In a known embodiment, the electric motor 5 has a hollow shaft through which the refrigerant is conveyed from the refrigerant line to the compressor 6. In this known embodiment, the electric motor 5 is therefore cooled by the refrigerant at most to a small degree.
In order to achieve efficient cooling of the motor-control device, the latter is arranged in close thermal contact with the refrigerant, preferably in the region between the outlet of the expansion member and the inlet of the compressor, that is in the region of low refrigerant temperatures. In a special embodiment, the refrigerant flows directly around the motor-control device. However, it is also possible, for example, to arrange the motor-control device outside the refrigerant flow on the housing of the compressor or on the refrigerant lines in good thermal contact with the refrigerant.
The method according to the invention for operating a compression-type refrigeration system of a vehicle is based on a refrigeration system which has a compressor, driven by an electric motor with motor-control device, a gas cooler, a controllable expansion member and an evaporator, and also a refrigerant circulation with a refrigerant. Due to the known mode of operation of such a compression-type refrigeration system, the latter has regions with a low temperature level and a higher temperature level.
According to the invention, provision is now made for the electric motor or the motor-control device to be cooled by the refrigerant in the region of the low temperature level. Furthermore, provision is made for the rate of flow of refrigerant through the expansion member to be controlled in such a way that the refrigerant at the inlet of the compressor has virtually constant superheating. Energy-efficient operation of the compression-type refrigeration system is achieved by this procedure. The electric motor and/or the motor-control device works in a region having a reduced temperature level, so that reduced losses occur at these elements. At the same time, virtually constant superheating of the refrigerant is set at the inlet of the compressor. The temperature increase or superheating increase of the refrigerant due to the waste heat of the electric motor and/or of the motor-control device is taken into account by this procedure. In this case, the superheating at the compressor inlet is set in such a way that so-called liquid shocks in the compressor may be ruled out.
Liquid shocks in the compressor are produced by refrigerant which is not evaporated one hundred percent. A liquid portion present in the gaseous refrigerant is not compressible. On account of the highly dynamic compression operation and the inertia of the outlet valves, liquid shock may therefore occur in the compression space. This effect may lead to the failure of the compressor.
To avoid liquid shocks, the superheating of the refrigerant at the compressor inlet is selected in such a way that there is slight superheating of, for example, 5-15 K. Although a high value for the superheating is admissible per se, this has an adverse effect on the efficiency of the complete unit.